Electroconductive polymers are known and enjoy increasing popularity, in particular also as material for producing electrodes for generating electric fields in various applications.
For example, an assembly for electrical arrangements is known from EP 0 307 007 B1, which comprises conductive components having different specific resistances. A resistor made of a conductive polymer, i.e. a mixture of an organic polymer and a conductive filler, which has a relatively high specific resistance at 23° C. of 1-500.000 ohm×cm, is here provided with a contact layer consisting of a conductive material which has a specific resistance that is lower than the specific resistance of the resistor, i.e. a specific resistance between 2.5×10−5 and 1×10−3 ohm×cm. The contact layer also consists of a conductive polymer which is doped with a metal, e.g. silver, or a carbon-based material, e.g. graphite. The contact layer is disposed onto the resistor in the form of band-like electrodes which interlock like fingers. Conductor rails are provided as contacting elements, which consist of a stretched net made of metal and which are folded around the contact layer and the electrodes formed therefrom, respectively. The area at the edges of the electrodes acts as contact area. Although, with this solution, the input resistance of the resistance layer is reduced by application of a contact layer having lower specific resistance, this contact layer itself consists of a doped polymer and hence has still a relatively high input resistance as well. This is particularly true if demixing close to the surface occurs when the contact layer is injection-moulded. Furthermore, the contact layer is here contacted via close-fitting conductor rails, i.e. stretched metal nets, and is thus not comparable to a dot-like contact. But there are many applications wherein dot-like contacting, e.g. via spring contacts, is necessary due to specific requirements or constructive conditions. But in this case, dot-like contacting of the disclosed contact layer would result in a burn-in of the contacting elements to the contact points when very high voltages would be applied.
Since metal ions are emitted from electrodes made of metal during electric discharge, particularly in the field of biological applications, electrodes made of conductive synthetic material are advantageous compared to commonly used metal electrodes. With the treatment of living cells, for example with electroporation or electrofusion, metal ions emitted into the respective cell suspension can either cause undesirable stimulation of the cells at lower concentrations or, at higher concentrations, act toxic on the cells. For instance, when cuvettes made of aluminium are used a negative effect due to the release of Al3+ ions could be demonstrated (Loomis-Hasselbee et al., Biochem J 1991, 277 (Pt 3), 883-885). Furthermore, if using cuvettes having electrodes made of metal generation of metal hydroxides or complexes of metal ions with biological macromolecules may occur (Stapulionis, Bioelectrochem Bioenerg 1999, 48(1), 249-254), what is often undesirable as well.
DE 102 08 188 A1 discloses containers with electrodes made of doped polymers. The doped polymers are polymers which are blended with conductive substances such as carbon fibers, graphite, carbon black (soot) or carbon nanotubes. Although those doped polymers have lower conductivity compared to intrinsically conductive polymers, it is a benefit that they are mouldable, i.e. that processing by the use of injection-moulding is possible. Thus, such doped polymers are variously useful and allow a cost-effective production of electrodes by injection-moulding. But it is a problem with such electrodes that demixing occurs during the injection-moulding process so that the concentration of the conductive dope is relatively low at the surface of the electrodes. Therefore, respective electrodes have a very high input resistance so that very high voltages have to be applied in order to achieve a sufficient current flow. But when usual dot-like contacting of these electrodes is used, for example via spring contacts, the contacts burn-in to the surface of the electrodes due to the high voltages applied so that the electrodes become unusable.
DE 101 16 211 A1 discloses a device for fusing living cells within an electric field, wherein the electrodes are also made of a doped synthetic material, i.e. a plastic material which is blended with carbon. The electrodes are connected to a voltage source via dot-like contact points and corresponding lead wires. Thus, also in this case it is a disadvantage that burn-in to the surface of plastic electrodes would occur if voltages should be applied, which are higher than those necessary for electrofusion. For example, to reach field strengths which are sufficient for certain applications in electroporation significantly higher voltages have to be applied to the electrodes. Field strengths of 2-10 kV/cm may be necessary, for instance, for the transfer of biologically active molecules into the nucleus of living cells. The voltage necessary to reach such field strengths would also in this case result in a burn-in of the contact points if the known polymer electrodes are contacted dot-like.
It is thus an object of the invention to overcome the existing deficiencies and to provide a method as identified above, which allows an effective reduction of the input resistance of the polymer within the contactable area in a simple and cost-effective manner. It is a further object of the invention to provide a formed body of the initially mentioned kind, which has a low input resistance, and which can be produced easily and cost-effectively.